Conventional wireless systems employ radio-frequency (RF) transmitters to produce an output signal that can be applied to an antenna for communication between stations separated by some distance. In mobile wireless networks, one station may be a subscriber station (SS), whereas another station may be a base station (BS). As the SS roams throughout the coverage area of the wireless network, the path loss between the SS and the BS changes due to a number of factors including the change in distance between the stations as well as the presence of objects in the environment that serve to obstruct or attenuate the signals traveling from one station to the other. To ensure proper network operation, the BS will instruct the SS to increase or decrease its transmit power as required to overcome the path loss between the SS and BS so that the BS will continue to receive the MS signals as channel conditions change. Over the full range of possible transmit powers, the SS must maintain a certain signal quality so as not to inhibit detection of its transmit signals by the BS. Depending upon the details of the physical environment between the SS and BS, at some critical distance from the BS the SS will no longer be able to increase its output power while maintaining the required signal quality. At that point, communication between the SS and BS can no longer be maintained and the link will be dropped unless the BS is able to hand-off communication with the SS to a neighboring BS. Therefore, the maximum output power capability of the SS is a critical parameter that ultimately determines the expected distance over which the SS and BS can communicate and thereby the number and spacing of BS sites that is required to provide reliable coverage in a mobile network. However, the greater the number of BS sites, the greater the cost to implement the mobile network.
Accordingly, there is a need to maximize the output power capability of the MS to ensure reliable coverage with a minimum of required BS sites. The coverage is usually limited by the MS as the BS transmitter typically has sufficient output power to provide reliable coverage over an acceptable cell area.
It is instructive to consider the factors limiting the maximum transmitter output power in a conventional RF transmitter. Among those factors are the error vector magnitude (EVM) and the spectral emissions mask. The EVM characterizes the fidelity of the actual transmit signal with respect to the intended transmit signal. This is commonly visualized as illustrated in FIG. 1 in which the complex transmitted signal comprising in-phase (I) and quadrature (Q) components at certain critical instants in time is compared to a regular constellation of points representing the ideal values of the transmitted signal at those same instants. The constellation of points that are used in transmission is referred to as the modulation. The EVM is given by the root-mean-square (RMS) distance between the actual signal and the corresponding ideal constellation points normalized to the average radius over all of the constellation points. Forward error correction codes are commonly used in wireless transmission. Taking together, the modulation and the coding schemes are referred to as the Modulation and Coding Scheme (MCS). Different Modulation and Coding Schemes have different EVM requirements. A greater EVM can be tolerated for a ‘loosely packed’ constellation corresponding than it can for a ‘densely packed’ constellation corresponding for the same coding rate. In many systems, the transmitter may be able to operate using a variety of MCS levels. Doing so allows for the transmission data rate to be adapted as conditions allow. For example, when the MS is closer to the BS, the BS will generally be able to detect a higher MCS level thereby allowing for an increased data rate for data transmitted from MS to BS. Similarly, when the MS is farther from the BS, the BS may need to reduce the MCS level to ensure reliable reception. Thus, having some flexibility to control the MCS level is advantageous in that it provides the ability to operate at the maximum data rate that can be accommodated by the link conditions. The transmitter EVM is degraded by noise and intermodulation distortion products produced by the transmitter as it amplifies the transmit signal.
A second factor limiting the maximum transmitter output power is the spectral emissions mask, which characterizes the amount of spurious emissions generated by the transmitter that fall into neighboring channels. As illustrated in FIG. 2, there is a limit on the acceptable level of such emissions to avoid interference with neighboring transmitters. These emissions are caused primarily by intermodulation distortion of the transmit signal occurring due to nonlinear amplification by the transmitter. Hence, both EVM and spectral emissions mask performance are determined by noise and nonlinearity in the RF transmitter.
A critical component in a conventional transmitter that produces such distortion is a power amplifier. A power amplifier will typically possess a maximum output power rating. Operating the power amplifier at output powers exceeding this rating may result in unacceptable EVM or spectral mask performance. As an RF transmitter may be asked to produce the maximum output power for any MCS level, it is generally necessary for the transmitter to comply with the most restrictive EVM requirement corresponding to the highest MCS level while also meeting the spectral emissions mask.
However, when the MS is positioned near the outer boundary of a given BS cell, the RF transmitter may be operating at a lower MCS level because a lower MCS level is more tolerant of attenuation along the path between MS and BS and therefore is easier to detect and demodulate. Under such operating conditions, one can infer based on the foregoing discussion that the maximum output power of the transmitter is primarily dictated by the spectral emissions mask requirement rather than the EVM requirement since the latter enjoys a relaxation for low MCS levels. However, a relaxed EVM requirement alone is not enough to permit operation of the transmitter at an increased output power because the transmitter must satisfy the tighter specification imposed by the spectral emissions mask requirement which is typically independent of MCS level.
It is desirable to have a technique that allows for increased output power at low MCS levels at the expense of EVM performance while maintaining a specified spectral emissions mask performance. Doing so would enable a beneficial increase in transmitter output power when the MS operates near its maximum range from the BS, thereby improving the reliability of the network and reducing the required number and spacing of base stations. An object of the present invention is to provide this capability.